This invention relates to the catalytic oxidation of methanol in the vapor phase to produce formaldehyde. More particularly it relates to a method for enhancing the effectiveness of the cooling step which follows the catalytic oxidation and in which the hot gases being discharged from the catalytic converter are cooled down to a temperature at or below which said gases can then be further processed to recover the contained formaldehyde by methods already known to those experienced in the art.
Many processes are known for catalytically oxidizing methanol vapor to formaldehyde, differing from one another in the number of reaction stages employed, in the nature of the catalyst, and in the ratio of oxygen to methanol in the reactant gases entering the catalytic conversion system. (Some processes employ a high oxygen:methanol ratio whereby a substantially complete conversion is obtained per reaction pass, while others employ a lower ratio of oxygen to methanol whereby a substantial portion of the methanol is not reacted and is subsequently recycled to the catalytic converter.) In some of the processes the reaction is conducted in a single stage while in others there are two reaction stages in sequence with the gases leaving the first stage being cooled and, if desired, modified as to their composition before being introduced into the second stage. Various catalyst are known, including silver gauze, silver crystals, silver on a suitable inert support, and several compositions comprising the oxides of various metals. The several processes vary among themselves also in that some of the catalysts are thought of as being essentially oxidation catalysts whereas others, including especially the more highly active metallic silver catalysts, are considered to catalyze both the oxidation and the dehydrogenation of the methanol.
Representative of the current oxidation technology is U.S. Pat. No. 3,959,383 to Northeimer, which describes the use of two reaction stages in sequence. Both stages use a silver catalyst, with the second stage catalyst being specifically silver crystals. The hot gaseous effluent from the first stage is cooled, and its composition adjusted before it is passed on to the second stage. The effluent from the second stage is then cooled again, and the cooled gases enter an absorption system for recovery of the contained formaldehyde by conventional methods. U.S. Pat. No. 2,462,413, to Meath, describes a process similar to that of Northeimer except that the catalyst is used on a support. U.S. Pat. No. 1,968,552, to Bond et al., describes a single-stage process using silver crystals. U.S. Pat. No. 2,504,402, to Field, describes a multiple-stage conversion system in which various catalysts can be employed, with iron-promoted molybdenum oxide being specifically exemplified.
The foregoing are but a sampling of the voluminous literature dealing with the catalytic conversion of methanol to formaldehyde, but it is to be understood that the present invention, to be discussed more fully below, has to do with the handling of the hot effluent gases from the catalytic converter and not with the mode of operation of the converter, nor with the catalyst or catalysts employed therein.
Regardless of variations in such factors as type of catalyst, number of reaction stages, and reactant proportions, all of the several reaction systems are operated with recognition of the fact that formaldehyde contained in the hot reaction product gases is subject to rapid post-reaction decomposition so long as the gases are hot (although the problem is less severe with the metal oxide catalyst, which operate at comparatively low temperatures). It is therefore standard practice to employ any of several expedients to reduce the temperature of the hot gases as rapidly as possible. For example, U.S. Pat. No. 2,908,715, to Eguchi et al., describes spraying water, methanol, or aqueous formalin directly onto the under surface of the catalyst layer in order to quench the hot gases as quickly as possible. Other variants of this spray-quench technique are also employed in which, although the liquid is not sprayed directly against the catalyst, it is still sprayed into the reaction chamber as close as possible to the downstream side of the catalyst. Other approaches include the use of ordinary shell-and-tube aftercoolers positioned as closely as possible to the catalyst bed as exemplified in British Pat. No. 1,131,380, to Farbenfabriken Bayer, in which the heart of the invention is the cooling of the reactor effluent gas to a temperature below 100.degree. C. in a time of less than 0.1 second. This is accomplished by keeping the zone between catalyst and catalyst condenser extremely small in volume. While not specifically disclosed in British Pat. No. 1,131,380, one feasible approach to minimizing the retention time of the hot gases between the catalyst and the condenser is to have the catalyst actually positioned on top of the tube sheet of the condenser or aftercooler, separated therefrom only by a spacer which prevents actual contact between the catalyst and the tube sheet of the condenser.
To recapitulate, it is well-known in the art to minimize the retention time of the hot reactor effluent gases between the catalyst and the aftercooler or condenser in order to lower the gas temperature as rapidly as possible. The present invention is not directed to the broad idea of effecting this rapid cooling but, rather, to a particularly efficacious method for doing so.
As will be seen, this invention deals primarily with a method for enhancing the heat-transfer characteristics of the aftercooler which follows the methanol-oxidation catalytic converter, the method comprising the use of a certain type of insert in the tubes of the aftercooler in order to bring about the desired heat transfer enhancement.
The use of inerts to enhance heat transfer is not in itself new. For example, a good over-all discussion is presented by A. E. Bergles in a paper titled "Enhancement of Heat Transfer" published in Vol. 6 of the Sixth International Heat Transfer Conference, pages 89-108, published by Hemisphere Publishing Corp., Wash. D.C., in 1978. There are also studies of heat transfer within packed beds of solids contained in tubes as exemplified by Calderbank et al., "Transactions of the Institution of Chemical Engineers" Volume 35 pages 195-207 (1957). Another treatment of the subject is provided by Schluender in Chapter 4 of "Chemical Reaction Engineering Reviews--Houston," ACS Symposium Series 72 (1978), which, like Calderbank et al., is concerned largely with catalytic reaction systems and the like rather than with the employment of tube inserts for the specific purpose of enhancing heat transfer. Finally, another system involving heat transfer in packed tubes is disclosed in U.S. Pat. No. 4,029,636, to Lowry et al., in which a hot gas containing molybdenum trioxide is passed through cooled tubes which are packed with ceramic balls in order to condense the molybdenum trioxide onto the surface of the balls so that it is removed from the gases and does not contaminate downstream processing equipment. The object is not to enhance the heat transfer rate but simply to condense out the molybdenum trioxide. No effort is made to provide rapid cooling of the gases as an end in itself, and enhancing the heat transfer as such is not taught by the patentees. Temperatures are quite high, even downstream from the cooled ceramic balls.
Although the enhancement of heat transfer by the use of inserts in the tubes of tube-and-shell heat exchangers is not new as explained above, it is not believed that this has ever been successfully attempted in cooling the gaseous effluent from a methanol-oxidation reactor. One reason is probably that metal inserts such as twisted metal ribbons that are known for heat transfer enhancement will also tend to catalyze the decomposition of hot formaldehyde vapor. This decomposition is discussed in, for example, the Kirk-Othmer "Encyclopedia of Chemical Technology" on page 233 of Volume 11 of the Third edition (1980) published by John Wiley & Sons. Chromia and alumina are mentioned in the same reference as being catalysts for the formaldehyde decomposition, and the alumina, as a known decomposition catalyst, would also cast a shadow on the idea of using non-metallic inserts for the same purpose. Broadly speaking, it appears that the art to date has considered, if it has considered at all the idea of using enhanced heat transfer for this purpose, that the combination of relatively high inlet temperatures along with adverse catalytic effects of the surfaces of whatever tube inserts might be employed, leads to an undesirable degree of formaldehyde decomposition. The result has been that the art heretofore has opted for either a direct quench using a cold liquid spray or, alternatively, empty cooling tubes made of a high alloy metal and cooled by a relatively low temperature liquid on the shell side.
One additional factor relating to the employment of enhanced heat transfer devices as contemplated by the prior art is that it has not generally been recognized that a high degree of heat transfer enhancement can actually be attained without, at the same time, suffering a significant pressure drop in the gas or other fluid passing through the tubes containing the heat transfer enhancement devices. For example, twisted metal ribbons, which do not normally cause a serious pressure drop problem, do not effect a very great improvement in heat transfer in the tubes in which they are installed. At the same time, packing the tubes with small solid bodies such as beads can result in a substantial obstruction to gas flow with resulting increase in pressure drop. Yet, in cooling the effluent gas from a methanol converter, it is not only necessary to effect rapid cooling for reasons of chemical efficiency but it is also necessary to avoid gas pressure drop as much as possible, because the converter itself is operated at low pressure (of the order of about 0.3-0.6 atmosphere gauge).
It is an object of the present invention to provide a method for rapidly cooling the effluent gas from a catalytic converter in which methanol is being converted to formaldehyde in the presence of molecular oxygen. It is another object to provide a method for effecting such cooling without the process complications inherent in the use of an internal liquid coolant. It is another object to cool the converter effluent gases more rapidly than in an ordinary shell-and-tube condenser or aftercooler of conventional design, with the result that post-reaction decomposition of the formaldehyde contained in the hot converter effluent gases, including a reversion reaction in which formaldehyde and hydrogen react to form methanol, is less than that which is experienced when using an ordinary shell-and-tube aftercooler. Other objects will be apparent from the following detailed description and examples.